In prior art, polyurethane powders are prepared by cryogenic grinding or extrusion and granulation of thermoplastic polyurethanes. (U.S. Pat. No. 3,214,411; JP-02-124978). JP 03-181526 discloses a process for producing polyurethane microparticles by dissolving polyurethane resin in amino alcohol at 130 to 150.degree. C. followed by cooling, filtering and drying under vacuum. In another method (JP 04-248875), hydroxyl and/or amino group containing resin is reacted with blocked isocyanates in a suitable solvent at elevated temperature. The solvent is removed under reduced pressure. The polyurethane powder obtained is filtered and dried. These techniques however, do not yield polyurethane powder in controlled particle size and shape.
Spherical polyurethane particles are prepared by suspension polymerization of an isocyanate terminated prepolymer in aqueous or non-aqueous medium. U.S. Pat. No.4,083,831 describes oil in water suspension polycondensation technique wherein an isocyanate terminated prepolymer is suspended in aqueous media. The reaction proceeds in aqueous medium and produces polyurethane urea powder which is stabilized by a combination of anionic and homopolymeric stabilizers. In an another variant, polyurethane spherical particles are obtained by suspension polycondensation in non-aqueous media (U.S. Pat. No. 4,032,516). In this art, the isocyanate terminated prepolymer is suspended in paraffin oil using sodium dodecylsulfosuccinate as a stabilizer. The chain extension was carried out with 1,4-butane diol in n-heptane and in the presence of an amphiphilic block copolymer namely 1-hexadecane vinylpyrrolidone as steric stabilizer.
Stabilization of polyurethane spherical particles in aqueous and non-aqueous medium is known in the prior art. For instance, JP 04-225755; JP 03-31359 and JP 04-202311 describes the use of a stabilizer derived from polycaprolactone, polybutadiene and isophrone diisocyanate to prepare polyurethane powder in n-heptane. In yet another application JP 02-38453 discloses the preparation of stabilizers derived from poly (butylene adipate), maleic anhydride and lauryl methacrylate. In addition to these block copolymers, protective colloids such as polyvinyl alcohol, methylcellulose, gelatin, sodium alkyl sulfate are also employed to stabilize polyurethane microspheres (JP 04-161416).
Polyurethane spherical particles obtained by this prior art are polydisperse and their size ranging between 1 and 1000.mu. even at higher stabilizer concentrations. Furthermore, none of the application uses diol and diisocyanate directly. Instead, an isocyanate terminated prepolymer is used to produce polyurethane microspheres.